In standard wireless communication systems, data is sent from a transmitter to a receiver by modulating the data onto a radio frequency (RF) carrier signal. In radio telephony, multiple users share a narrow range of frequencies assigned by prior agreements. Because spectrum space is severely limited, the modulation techniques chosen for digital telephony are of crucial importance, and are designed to transmit the highest amount of data within the narrow confines of a small sub-range of the total spectrum allocated for a particular service provider.
The Global System for Mobile (GSM) telephony prescribes standards for modulating data from multiple users onto a composite RF carrier. In GSM, packages of digital data are sent in modulated periods of RF power called “bursts”, transmitted at regularly-spaced intervals during a sequence of “time slots”, allocated to different users (at different carrier frequencies). The power of the transmitted carrier remains constant during each burst, because the data is modulated onto the carrier using a constant-amplitude technique that varies only its phase.
FIG. 1 illustrates a typical GSM burst. The outer boundaries 1 and 2 of the burst form a conceptual “envelope” within which the high-frequency (usually between 0.8 and 2.2 GHz) carrier signal 3 is modulated. The amplitude of the signal begins at essentially zero and ramps up during an initial period of about 20 microseconds (μs). The amplitude is then held constant for about 560 μs after which it is ramped back down to zero. The controlled-rate ramp-up and ramp-down portions of the signal are included to prevent interference between adjacent channel frequencies. If the power level of each burst increased or decreased almost instantaneously to its final value, spurious high-frequency harmonics would be generated, causing significant power to appear outside of the band assigned to a certain channel. In severe cases, the power transmitted in adjacent channels, or even channels at more remote frequencies, would exceed the limits prescribed by the regulatory body (for example, the FCC in the USA).
During the data burst interval, the transmitted power must be accurately controlled to conform to GSM and relevant regulatory standards. If the signal is transmitted with too little power, the link between the transmitter and receiver will be lost, when the bit error rate (BER), caused by noise in the receiver, exceeds a limit above which error correction is no longer possible. If the signal power is too high, it may interfere with adjacent channels (due to receiver selectivity limitations), or violate regulatory limits, or reduce “talk time” in a battery-powered handset. Thus, the power control in either a base-station or handset transmitter demands the use of very accurate circuitry.
FIG. 2 illustrates a prior art system for controlling the power of an RF telephony system. A power amplifier (PA) 10 drives an antenna 12 with an RF output signal which it generates by amplifying a lower-power RF carrier signal that has data modulated thereon. The amount of power gain is controlled by an Automatic Power Control signal VAPC. A directional coupler 14 extracts the signal RFIN which is an attenuated sample of the output power of the PA flowing toward the antenna. The control system 16 includes a detector 18 which transforms this RF sample to a corresponding baseband signal VMEAS, to provide a measurement of the power delivered to the antenna. The detector can use a simple diode circuit, a logarithmic amplifier (log amp), an amplitude-squaring cell, which may or may not be preceded by an amplifier, or other suitable detector. A comparison element 20 generates an error signal IERR which represents the difference between the actual signal measurement, VMEAS, and the “set-point” signal, VSET, which determines the desired antenna power. This deviation between the signal representations of actual and desired power, in this example assumed to be in current form, is integrated by a capacitor C1 and buffered by an amplifier 22, usually having a small non-inverting gain, which provides the power-control signal, VAPC, for use by the PA. The comparison element, integrating capacitor and buffer amplifier can collectively be viewed as an error-nulling circuit.
The set-point signal VSET is an independent variable, accurately scaled to conform to the design parameters of the overall detector sub-system. For example, if the detector is based on a logarithmic amplifier (log amp), the design parameters are the logarithmic slope and intercept. Whenever a certain VSET is applied to the set-point input, the control loop adjusts the output from the PA until the input to the detector corresponds very closely to the demand level represented by the set-point signal. On the other hand, the signal generated by the detector system, VAPC, which controls the PA, is the buffered output from the error integrator. VAPC assumes the generally imprecise value needed by a variety of PA types to provide the power required to satisfy the set-point demand, as represented by RFIN.
The generalized detector system of FIG. 2 is readily adaptable for use in RF power control for GSM modulation. A log amp is especially suited as the detector element, since it provides a wide measurement range (typically up to 60 dB, which is a 1000:1 range in transmitted power). Being essentially an envelope detector, it responds to the modulation envelope of the data bursts. In GSM telephony, the aforesaid ramping process is controlled within the transmitter, usually by generating a time-sequence of values in software which are transformed into a voltage waveform using a Digital-to-Analog Converter (DAC). Since the system of FIG. 2 is designed to maintain closed-loop power control continuously, from the beginning of the ramp-up, throughout the data burst interval, and during the ramp-down, it is well-suited to the basic GSM modulation format, in which the RF signal has a constant power amplitude during the burst.